Sphingolipids are abundant components of the plasma membrane of higher eukaryotes. Specific sphingolipids are produced via the addition of polar groups to the 1-hydroxyl group of ceramide. The mechanism of de novo synthesis of ceramide in plants begins with the condensation of serine and palmitoyl-CoA to yield 3-ketosphinganine. This reaction is catalyzed by serine palmitoyltransferase (SPT; EC 2.3.1.50). Further reactions convert 3-ketosphinganine to ceramide. Serine palmitoyltransferase is thought to have at least two subunits, Lcb1 and Lcb2, with Lcb1 containing a conserved non-functional pyridoxal binding site which does not have the essential lysine residue.
Sphingolipids appear to play essential roles in cellular events such as proliferation, senescence, differentiation, apoptosis and response to desiccation stress. Although not much is known about the mechanism of this pathway, it has been suggested that SPT is the rate-limiting step in the production of ceramide. The genes encoding the Lcb1 subunit of SPT have been isolated from human, mouse and Saccharomyces cerevisiae. All of these genes appear to contain a putative transmembrane domain and are probably localized at the endoplasmic reticulum (Hanada, K. et al. (1997) J Biol Chem 272:32108-32114). Accordingly, the availability of nucleic acid sequences encoding all or a portion of this enzyme would facilitate studies to better understand the de novo synthesis of ceramide, provide genetic tools for the manipulation of this biosynthetic pathway, and provide a means to control sphingolipid concentration in plant cells.